Photocatalytic C-C Coupling by a Au(I) Complex: Mechanistic Elucidation and SET Modulation

Photocatalytic C-C coupling reactions mediated by molecular complexes represent an advancing field of research, in which Au-based catalysts have demonstrated significant potential. Herein, we employ an integrated computational strategy: incorporating MS-CASPT2, DFT, and kinetic analyses of fluorescence, phosphorescence, intersystem crossing, internal conversion, and electron transfer processes to unravel the mechanistic intricacies between aryldiazonium salts and alkynyl-silanes catalyzed by a naphthalene-di-imide-functionalized N-heterocyclic carbene Au-(I) complex, [(NDI-NHC)-Au-Cl]. Upon photoexcitation, a four-state model (S₀, S₁, T₂, and T₁) of the photophysical process is identified. The reaction proceeds via a triplet-state-initiated tandem comprising single electron transfer (SET), N₂ extrusion, radical addition, and intersystem crossing, culminating in reductive elimination to complete the catalytic cycle. Furthermore, introducing electron-withdrawing groups such as the cyano group into the aryldiazonium scaffold enhances SET efficiency by a certain cancellation of the reorganization energy by driving energy, thereby reducing the free energy barrier. The nonradiative decay dominates the reaction coordinates. Leveraging these mechanistic insights, the SET modulation strategy is theoretically extended to Ag and Cu complexes, which exhibit comparably high performance in C-C coupling reactions along with improved cost-effectiveness. This work not only establishes fundamental structure-reactivity relationships in Au-photocatalyzed cross-couplings but also provides a general framework for optimizing photoinduced electron transfer processes.